It has become critical to collect information about the liquid level in wells for a variety of reasons. These may include the ability to manage water resources, monitoring civil engineering structures such as dams or buildings, and various earthworks such as bridges, roads, landfills, etc. It is important to determine the actual fluid level and have the ability to monitor fluid level changes over time.
A number of techniques have been invented and commercialized over many decades. As disclosed in U.S. Pat. No. 5,027,655 issued Jul. 2, 1991, the most common method to determine the fluid depth in a well or borehole involves lowering a measuring tape down the wellbore. When the end of the tape contacts the fluid surface, a change in the impedance between two contacts at the end of the tape provides an indication that the fluid surface has been reached, whereupon the fluid depth can be read from the demarcations on the tape. This approach is fraught with uncertainty, since the tape can encounter wet surfaces as it is lowered down the borehole, or become entangled and hung up on protrusions and structures within the borehole, resulting in grossly erroneous readings.
A second method involves lowering a gas-tight tube down to slightly below the fluid surface in the borehole. The pressure in the tube is then increased until bubbles begin to exit at the distal end of the tube. By detecting the presence of bubbles, the pressure required to create the bubbles is measured and, together with the known length of the tubing, used to estimate the depth of the fluid. This method suffers from problems similar to the above described measuring tape technique.
A third method involves introducing a sound pulse at the top of the borehole and directing it to the fluid surface at the bottom of the borehole. By measuring the Time Of Flight (TOF) between launching the pulse and detecting the pulse that reflects from the fluid surface, and knowing the speed of sound in the borehole, the depth of the fluid surface can be estimated. A number of techniques have extended this approach. For example, U.S. Pat. No. 4,934,186 issued Jun. 19, 1990 discloses the detection of reflections from known, regularly spaced collars along the borehole to provide calibration signals for the TOF measurement to the fluid surface. In U.S. Pat. No. 4,389,164 issued Jun. 21, 1983 the inventors disclose the use of a TOF acoustic pulse measurement system to control a pump and thereby maintain a desired fluid level in the borehole. U.S. Pat. No. 4,318,298 issued Mar. 9, 1982 uses an acoustic TOF system to monitor the fluid level in a borehole on a periodic basis.
This approach suffers from false reflections that can result from a plurality of sources, including protrusions, changes in bore diameter, abrupt changes in borehole direction, changes in borehole wall composition, and resonant effects that can occur between one or more of the above-mentioned perturbations.
A fourth technique involves the use of a continuous tone, frequency modulated (frequency chirped) acoustic signal directed down the borehole. The reflected acoustic signal is detected and mixed with the launched signal, and the resulting difference frequency is proportional to the round trip time of flight of the acoustic signal from the top of the borehole to the fluid surface. Alternatively, the continuous tone is frequency modulated in order to maximize the amplitude of the resulting detected signal. In this instance, an acoustic resonant cavity is formed in the borehole between the acoustic source and the fluid surface. Knowledge of the speed of sound in the borehole can be used to estimate the acoustic wavelength of the resonant frequency and thereby the fluid depth. This approach is limited by the need for a significant frequency chirp of a low frequency signal, which can limit the range of borehole depths that can be interrogated.
A fifth technique involves creating an acoustic pulse at the top of a borehole that consists of series-connected casings with identical lengths. The reflected acoustic signals are detected and are comprised of a plurality of reflections from the regularly spaced collars along the borehole, as well as the reflection from the fluid surface. By using the known spacing between collars to calibrate the speed of sound in the borehole, the distance to the fluid surface can be estimated. This approach finds limited utility for boreholes that have contiguous, series-connected casing sections between the top of the well and the fluid surface.